Sputtering technique

ABSTRACT

THE ELECTRICAL AND PHYSICAL PROPERTIES OF CATHODICALLY SPUTTERED FILMS MAY BE CONTROLLED BY UTILIZING EITHER ELECTRON OR NEGATIVE ION BOMBARDMENT OR ALTERNATIVELY, ELECTRON OR NEGATIVE ION, AND POSITIVE ION BOMBARDMENT OF   SUBSTRATE SURFACES DURING THE COURSE OF THE SPUTTERING PROCESS.

June 29, 1971 SCHWARTZ ETAL 3,589,994

SPUTTERING TECHNIQUE 2 Sheets-Sheet 2 Filed Ja 3 31, 1968 0 M m M in u 1M. 1% m F m IN A m TILITLTIF' .w a 225.5%. 5351 SUBSTRATE POTENTIAL FA/G. 5

OXYGEN PARTIAL PRESSURE -TORR.

United States Patent 3,589,994 SPUTTERING TECHNIQUE Newton Schwartz, Morris Township, Morris County, and Frederick Vratny, Berkeley Heights, N.J., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, NJ.

Continuation-impart of abandoned application Ser. No. 372,537, June 4, 1964. This application Jan. 31, 1968, Ser. No. 701,964

Int. Cl. C23c 15/00 US. Cl. 204-192 7 Claims ABSTRACT OF THE DISCLOSURE The electrical and physical properties of cathodically sputtered films may be controlled by utilizing either electron or negative ion bombardment or alternatively, electron or negative ion, and positive ion bombardment of substrate surfaces during the course of the sputtering process. I

This application is a continuation-in-part of copending application Ser. No. 372,537, filed June 4, 1964, now abandoned.

The present invention relates to a technique for the deposition of thin films by cathodic sputtering techniques. More particularly, the present invention relates to a technique for the growth of thin films of controlled electrical and physical properties by a novel cathodic sputtering procedure.

In recent years interest has been expanded in thin film metal electrical resistors and the preparation of such films by cathodic sputtering techniques. Unfortunately, it has frequently been found that the specific resistivity and temperature coefiicient of resistance of certain of these films are prone to variability and non-uniformity. These difiiculties have been controlled to a limited extent by painstaking control of background pressures, leak rates and deposition parameters. However, variability is still found in the quality of the deposited film.

In accordance with the present invention, a technique for the deposition of thin films by cathodic sputtering is described wherein the electrical and physical properties of deposited films are controlled by utilizing either (a) electron or negative ion bombardment or (b) alternatively, electron or negative ion, and positive ion bombardment of substrate surfaces during the course of the sputtering process. The inventive technique involves sputtering in a three electrode system including an anode member, a cathode member, and a ground or reference electrode, wherein the anode member is biased either with a positive or alternating potential of at least 0.1 volt with respect to the reference electrode, the cathode being biased at conventional potentials with respect to the reference electrode. In the operation of the sputtering process, the substrate member is physically situated upon the anode member and assumes the potential thereof when a difference of potential is impressed between anode and reference electrode, thereby resulting in the desired bombardment of the substrate surfaces.

The described technique has been found to result not only in control of film parameters such as resistivity and temperature coefiicient of resistance but also in the control of physical properties, namely, crystallographic phase and chemical composition. Thus, for example, sputtering of tantalum in accordance with the inventive procedure may be eifected to yield either beta tantalum or body centered cubic tantalum of predetermined resistivity and temperature coeflicient of resistance, a significant advantage from a device standpoint. Furthermore, the degree 3,589,994 Patented June 29, 1971 of film perfection is susceptible to control by the described technique.

In an alternative embodiment, it has been determined that the technique is equally efficacious in a reactive ambient. Therefore, cathodic sputtering of thin films in the presence of nitrogen, oxygen, and so forth, may also be effected by operation in the described manner.

The described technique departs from conventional diode sputtering by the use of a reference electrode maintained at ground or a fixed potential and with respect to which both the anode and cathode members are biased.

For convenience, the electrode system described herein will hereinafter be designated as follows:

First electrode=reference electrode Second electrode=anode Third electrode=cathode It will also be understood by those skilled in the art that in the operation of the process, the substrate member upon which the thin film of interest is to be deposited is physically situated upon the second electrode and assumes the potential thereof.

The invention will be more readily understood by reference to the following detailed description, taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a front elevational view, partly in section, of an exemplary apparatus suitable for the practice of the present invention;

FIG. 2 is a graphical representation on semi-log c0- ordinates of specific resistivity in microhm-centimeters and temperature coefficient of resistance in p.p.m./ C. against applied substrate potential in volts showing variations in the noted parameters and structure as a function of anode bias potential for sputtered tantalum films prepared in accordance with the present invention;

FIG. 3 is a graphical representation on linear coordinates of specific resistivity in microhm-centimeters and temperature coefficient of resistance in p.p.m./ C. against the ratio of applied alternating current and cathode D-C current showing variations in the noted parameters as a function of anode-cathode current ratio and structure for sputtered tantalum films prepared in accordance with the present invention;

FIG. 4 is a graphical representation on semi-log coordinates of percent relative imperfection against applied substrate potential in volts showing variations in film perfection as a function of substrate bias for sputtered boo-tantalum films prepared in accordance with the present invention; and

FIG. 5 is a graphical representation on semi-log coordinates of specific resistivity against oxygen partial pressure showing variations in resistivity as a function of oxygen partial pressure for reactively sputtered tantalum films prepared in an oxygen ambient in accordance with the present invention, the substrate having applied positive potentials of 0.1, 5, and 50 volts D-C.

With reference now more particularly to FIG. 1, there is shown a vacuum chamber 11 provided with an outlet 12 for connection to a vacuum pump (not shown), an inlet 13 for the introduction of a suitable sputtering gas, and a base plate 14 which acts as the first electrode for sputtering. Shown disposed within chamber 11 is a substrate holder or second electrode 15 and a third electrode 16, the latter being comprised of the material which is required to be deposited upon substrate member 17. Third electrode 16 is connected to the negative pole 18 of a direct current high potential supply, the positive pole of which is connected to base plate 14 (as at 19), the latter being connected to ground bias or permitted to float. Second electrode 15 may be connected to (a) a source of alternating current 20, one side of which is connected to base plate 14 (as at 23), or (b) the positive pole 21 of a direct current source, the negative pole of which is connected to base plate 14 (as at 23).

The present invention may conveniently be described by reference to an illustrative example wherein it is desired to cathodically sputter any of the well-known film forming metals, for example, tantalum, niobium, titanium, zirconium, aluminum, et cetera, in an apparatus of the type shown in FIG. 1.

Substrate 17 is first vigorously cleaned. Conventional cleaning agents are suitable for this purpose, the choice of a particular one being dependent upon the composition of the substrate itself. Substrate 17 is placed upon substrate holder 15 as shown in FIG. 1, the latter being composed of a suitable conductor, for example, tantalum, stainless steel, et cetera.

The vacuum techniques utilized in this invention are known (see Vacuum Deposition of Thin Films, L. Holland, I. Wylie & Sons, -Inc., New York, 1956). By this process the vacuum chamber is first evacuated, flushed with an inertgas, as, for example, any of the members of the rare gas family such as helium, argon or neon, and the chamber then re-evacuated. The extent of the vacuum required is dependent upon consideration of several factors which are well known to those skilled in the art. However, for the purposes of the present invention, a practical intial pressure range is 10 to 10 torr, while suitable inert gas pressures during sputtering range from 0.5 to 100x 10- torr.

After the requisite pressure is attained, third electrode 16, which may be composed of any of the above-noted film-forming metals or, alternatively, may be covered with any of the film-forming metals, for example, in the form of a foil, is made electrically negative with respect to base plate 14.

The minimum voltage necessary to produce sputtering is dependent upon the particular film-forming metal employed. For example, a direct current potential of approximately 1000 volts may be employed to produce a sputtered layer of tantalum suitable for the purposes of this invention, minimum voltages for other film-forming metals being well known to those skilled in the art. However, in certain instances it may be desirable to sputter at voltages greater than or less than the noted voltage.

The next step in the inventive procedure involves applying a potential either continuously or intermittently to substrate holder and substrate 17 whereby they are made electrically positive with respect to base plate 14. This end may be attained by applying either (a) a positive direct current potential or (b) an alternating current potential to holder 15 by conventional means.

It has been determined that if a potential of at least 0.1 volt D-C and ranging up to +500 volts be applied to substrate holder 15 with respect to the base plate, the surface bombardment of substrate 17 by species in the glow discharge may be precisely controlled, thereby permitting control of the properties of the deposited film. Alternatively, an alternating current potential ranging from 0.1 to 5000 volts may be applied to the ungrounded substrate holder with respect to the base plate, so attaining similar results.

The spacing between the substrate holder (anode) and cathode is not critical. However, the minimum separation is that required to produce a glow discharge. For the best efficiency during the sputtering process, the substrate should be positioned immediately without the well known Crookes Dark Space.

The balancing of the various factors of voltage, pressure and relative positions of the cathode and substrate holder to obtain a high quality deposit is well known in the sputtering art. However, it will be appreciated that the main impact of the present invention lies in the discovery that applying a specific bias, either a positive direct current or alternating current, to substrate holder 15 and substrate 17 during sputtering permits control of film parameters.

With reference now more particularly to the example under discussion, by employing a proper voltage, pressure and spacing of the various elements within the vacuum chamber, a layer of a film-forming metal is deposited upon substrate 17. The sputtering is conducted for a period of time calculated to produce the desired thickness.

With reference now more particularly to FIG. 2, there is shown a graphical representation on semi-log coordinates of resistivity in microhm-centimeters and temperature coefiicient of resistance in p.p.m./ C. against applied potential in volts showing the properties obtained by sputtering tantalum in the apparatus shown in FIG. 1 employing a third electrode potential of 5 kilovolts with respect to the first electrode, an argon pressure of 10 microns and various positive direct current biases at the substrate holder and the substrate with respect to the first electrode.

It is noted that with the substrate holder essentially at zero potential (indicated as +0.1 volt in the logarithmic voltage scale of the figure) the deposited film evidenced a resistivity of approximately 200 microhmcentimeters and a temperature coeficient of resistance of approximately 0 p.p.m./ C. As the substrate is biased more positively, both the resistivity and temperature co-- efiicient remain substantially constant until approximate-- 1y 10 volts, at which point the resistivity decreases to a low of 50 microhm-centimeters at 50 volts and the temperature coeflicient increases to a high of approximately +1600 p.p.m./ C. at volts. With increasing bias, it is noted that this trend is reversed. It may also be noted that control of substrate potentials at values below 10 volts results in the formation of beta tantalum, whereas increasing the bias beyond 10 volts results in the formation of body centered cubic tantalum.

With reference now to FIG. 3, there is shown a graphical representation on linear coordinates of resistivity in microhm-centimeters and temperature coefficient of resistance in p.p.m./ C. against anode cathode current ratio showing the properties obtained by sputtering tantalum in the apparatus shown in FIG. 1 employing a cathode potential of 5000 volts direct current, an argon pressure of 10 millitorr and various anode to cathode current ratios wherein an alternating bias is applied to the anode.

It is noted that the use of an anode to cathode current ratio of 02 results in the deposition of a film evidencing a resistivity of approximately 42 microhm-centimeters and a temperature coefficient of +1000 p.p.m./ C. As the anode to cathode current ratio increases, the resistivity is increased while the temperature coefficient decreases rapidly whereas decreasing the anode to cathode current ratio results in a similar trend.

It will be appreciated, therefore, that it is possible to adjust the resistivity and temperature coefficients as well as structure of the deposited films by appropriate substrate biases.

With reference now to FIG. 4, there is shown a graphical representation of relative film imperfection (relative internal stress in the film) as a function of substrate potential for b.c.c.-tantalum films deposited in accordance with the invention. As noted, with the substrate essentially at zero potential (indicated as :0.1 volt in the logarithmic voltage scale of the figure) the degree of perfection is approximately 25 percent, and with increasing positive bias the degree of perfection decreases to approximately 0 percent (perfection being determined by usually counting the number of holes and cracks in the film). However, negative biasing ultimately results in a dramatic increase in imperfection level to a high of 100 percent, at which point the films are of no interest from a device standpoint.

FIG. 5 is a graphical representation showing specific resistivity as a function of oxygen partial pressure in torr for tantalum films reactively sputtered in the presence of oxygen in accordance with the present invention, the substrate member being biased at substrate potentials of 0.1, 5, and 50 volts. It will be noted from the graph that over the range of oxygen partial pressures of approximately 10 10 torr the specific resistivity of the deposited films may be controlled over a range of from 30 to 10,000 microhm-centimeters at the various applied biases. Additionally, it has been found that the temperature coetficient of resistance of certain portions of the described curves are within the range from 150 to 300 p.p.m./ C., a desirable property from a device standpoint.

Several examples of the present invention are described in detail below. These examples are included merely to aid in the understanding of the invention, and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE 31 This example describes the preparation of a sputtered tantalum film.

A cathodic sputtering apparatus similar to that shown in FIG. 1 was used to produce tantalum films. In the apparatus employed, the base plate 114 was grounded, the cathode was biased at 5 kilovolts negative with respect to ground, and the substrate holder and substrate were biased volts positive with respect to ground.

A glass microscope plate was used as the substrate. The slide was washed in a nonionic detergent, boiled in hydrogen peroxide and dried. The tantalum was employed in the form of an arc melted ingot slab as the cathode of the apparatus.

The vacuum chamber was initially evacuated to a low pressure of the order of 10- torr, flushed with argon and re-evacuated to 10 millitorr.

The substrate holder and cathode were spaced approximately 3 inches apart, the substrate being placed on the former. A direct current voltage of 5000 volts was impressed between cathode and base plate, and a direct current voltage of +5 volts was impressed between substrate holder and substrate 17, and base plate 14.

Sputtering was conducted for 60 minutes, so resulting in a beta-tantalum film 5000 A. thick and evidencing a resistivity of approximately 200 microhm-centimeters and a temperature coefficient of resistance of approximately -50 p.p.m./ C.

EXAMPLE II The procedure of Example I was repeated with the exception that an alternating bias of 250 volts was impressed between the ground and the substrate holder and substrate, so resulting in an anode-cathode current ratio of 0.2 and so resulting in a tantalum film evidencing a resistivity of approximately 42 microhm-centimeters and a temperature coeflicient of resistance of 1000 p.p.m./ C.

What is claimed is:

1. A method for the deposition of thin films of controlled electrical and physical properties upon a substrate by cathodic sputtering in a vacuum chamber in which there is a first electrode, a second electrode having a substrate positioned thereon, and a third electrode which comprises the steps of evacuating the said vacuum chamber, admitting a sputtering gas into the said vacuum chamber, biasing the said second electrode and substrate at a fixed positive potential within the range of 0.1-500 volts with respect to said first electrode and biasing the said third electrode negative with respect to said first electrode and said second electrode at a potential distinct from that of the said second electrode, said first electrode being maintained at a fixed potential, thereby efiecting sputtering at said third electrode, deposition of a thin film of sputtering material upon said substrate and electron bombardment and alteration of the properties of said thin film.

2. A method in accordance with the procedure of claim 1 wherein said first electrode is maintained at ground potential.

3. A method in accordance with the procedure of claim 1 wherein said thin film is tantalum.

4. A method in accordance with the procedure of claim 1 wherein said thin film is deposited by reactive sputtering in the presence of oxygen.

5. A method in accordance with the procedure of claim 3 wherein said thin film is beta-tantalum.

6. A method in accordance with the procedure of claim 3 wherein said thin film is body centered cubic tantalum.

7. A method in accordance with the procedure of claim 1 wherein said thin film is deposited by reactive sputtering in the presence of nitrogen.

References Cited UNITED STATES PATENTS 2,164,595 7/ 1939 Siebertz 204-192 3,021,271 2/ 1962 Wehner 204-192 3,258,413 6/ 1966 Pendergast 204-192 3,394,066 7/1968 Miles 204-164 3,324,019 6/ 1967 Laegreid et al 204-192 3,361,659 1/1968 Bertelsen 204-192 3,391,071 7/ 1968 Theuerer 204-492 FOREIGN PATENTS 939,275 10/1963 Great Britain 204-192 OTHER REFERENCES A.P.C. application of Berghaus, Ser. No. 283,312, published May 18, 1943.

HOWARD S. WILLIAMS, Primary Examiner S. S. KANTER, Assistant Examiner 

